Electrocardiographic waveform analyzer

ABSTRACT

There is disclosed a system for reviewing ECG signals at high speed. Successive ECG waveforms are superimposed on each other in a display, but the rate at which traces of the waveforms are formed is independent of the rate at which ECG waveforms are played back from a tape on which they are recorded. This is achieved by storing data representative of each ECG waveform in a recirculating memory whose recirculation rate is faster than the input data rate, and using the recirculated data to form the display. A similar recirculating memory is provided to form a display of each 4-second ECG signal segment which contains a premature beat, this display being formed automatically without requiring operator control.

United States Patent 1191 Harris et al. Apr. 1, 1975ELECTROCARDIOGRAPHIC WAVEFORM 3,718.772 2/1973 Sanctuary 128/206 AANALYZER Primary Examiner-William E. Kamm [751 $333kl ggir fg ml 'lfa ofAttorney, Agent. or Firm-Joel Wall; William c. Mass. Nealon; H. R.Berkenstock, Jr.

[73] Assignee: American Optical Corporation,

Southbridge, Mass. 11 d l d ABSTRfACT ECG I ere 1s |sc ose a s stem orreviewmg s1 na 5 [22] F'led: 1974 at high speed. Succesiive ECGwaveforms are ssper- [2|] Appl. No: 461,042 imposed on each other in adisplay. but the rate at which traces of the waveforms are formed isindependent of the rate at which ECG waveforms are played [52] Cl128,106 23/206 260/54 back from a tape on which they are recorded. Thisis [51] hit. CI A61) 5/04 achieved y Storing data representative of eachECG [58] Fleld Search waveform in a recirculating memory whoserecircula- 28/206 V; 360/6 54 tion rate is faster than the input datarate, and using the recirculated data to form the display. A similar re-[56] References cued circulating memory is provided to form a display ofUNITED STATES PATENTS each 4-second ECG signal segment which contains a1698,42? lit/I954 Steele 360/54 premature beat. this display beingformed automati- 3,267.933 8/1966 Mills ct al l28/2.06 A cally withoutrequiring operator control. 3.587.564 6/197] Hagan et al.. l28/206 R3.59811 10 8/1971 Edmark 128/206 A 9 Clams, 7 Dtawlng Figures ERJEHTED 1SiiZET 2 BF 5 was: 5 2 53:52. 3.55:: :55 h. s; 5352; \2: IE Z5 is z N: 3E12 Em n n u E: on n n u n on 523 ZN Emir a a .z. 2. 2 dam w P m N 5:5.5 J: r :N 5 i 3; g

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ATENTH] APR 1 1975 ELECTROCARDIOGRAPHIC AYEFORM ANALYZER This inventionrelates to the analysis and display of electrocardiographic waveforms.and more particularly to such analysis and display at high speeds.

In the field of continuous ECG recording. a patient is often providedwith a small. portable tape recorder: standard electrodes are attachedto the patient's chest and leads are connected to the tape recorder. Acontinuous ECG signal. extending to er as long a period as 24 hours. maythus be recorded on a standard tape cassette without in any wayconfining the patient. Rather than to require a physician or technicianto review a 24-hour tape in "real time (which would require 24 hours ofeffort). techniques ha\e been de\ised for speeding up the reviewprocedure.

In Holtcr et al. US. Pat. No. 3.25.136 issued on Nov. 2. I965 amlentitled "Electrticardiographic Means". one such technique is disclosedThe ECG signal recorded on the tape is played back at a much greaterspeed than that used during recording; for esantple. a tape may beplayed back (it) times as fast. The ECG signal is applied to thevertical deflection plates of an oscilloscope. and the horizontal sweepis triggcred by each ECG waveform What results in the case of a normalsignal is a series of similar superimposed waveforms displayed on thescreen. The waveforms "blend" into each other and what is iewed is forall intents and purposes a single ECG waveform which is smeared slightlyin view of small differences from cycle to cycle. Whenever an "unusual"leg. premature) waveform signal is applied to the vertical deflectionplates. the resulting trace is different? This is an indication of anabnormal waveform. The reviewer may then slow down the tape, and playthe signal back at slower speed while a paper trace is made of thewaveforms in the sequence of interest.

A continuous bright (normal) waveform is formed on the display providedthat the combination oftape speed and the persistence of the phosphor onthe face of the CRT are great enough to allow one waveform to blend ormerge into another. This can be understood by considering the kind ofdisplay which is formed when the tape is played back at normal speed.with the horizontal sweep being synchronized to the waveforms (that is.there being approximately l horizontal sweep per second. correspondingto a (10 beat-per-minute heart rate). In such a case. the electron beamsweeps across the face of the tube once each second, and the waveformwhich is displayed is ofthe well-known bouncing ball type. Unless thepersistence of the phosphor is very long. a complete ECG waveform is notcontinuously visible. Only when the tape is played back at greater speeddo the superimposed successive ECG complexes result in a continuousbright image.

But if the persistence of the phosphor is too high. then with a highframe rate ((10 per second) there can be considerably smearing of thedisplay. On the other hand. if the phosphor has low persistence, even a60- per-second frame rate will result in flicker. The basic problem inthe design of such a system is that the quality of the image isnecessarily dependent on the tape speed.

It is an object of our invention to provide a system for analyzing ECGwaveforms at high speed by forming superimposed traces ofthem on thescreen of a CRT, but

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in which the quality of the image thus formed is independent of tapeplayback speed.

Another object of our invention is to control the automatic continuousdisplay of a short ECG signal segment whenever an abnormal condition isdetected. in order to insure that no such conditions are missed. Eachsegment thus displayed remains on the screen until another is detected.This allows the technician or physician reviewing the \vmeforms toe\amine a short segment at his leisure. tThe tape can men he stoppedduring this time period. without either trace disappearing from the faceof the CRT: the last ECG waveform which was read from the tape remainsat the top of the display. and the last 4-second segment containing anabnormal waveform remains at the bottom of the display.)

In accordance with the principles of our imention. each analog ECG w a\eform read from the tape is sampled and digital representations of thesamples for a complete waveform are temporarily stored. When the samplesrepresentathe of a complete waveform are available. they are quicklytransferred to a recirculating shift register [recirculating memory).Successive samples at the output of this recirculating shift registerare converted to analog form and used to form a single ECG wateform atthe top of the CRT display. The hori zontal sweep ofthe CRT issynchronized to the recirculation time of the shift register. Theimportant point here is that the recirculation rate (CRT horizontalsweep time) is completely independent of the rate at which ECG waveformsare read from the tape. t In fact. even if the tape is stopped. therecirculating data continues to be used to form a display of the lastwavcform.)

While ECG waveforms may be read from the tape at a rate of 6|) persecond. the CRT frame rate. that is. the rate at which ECG waveforms areformed on the display. may be in the hundreds per second. This meansthat each individual waveform may be traced out on the screen severaltimes prior to the storage of a new waveform in the recirculating shiftregister. Smearing of the continuous bright image can be avoided byusing relatively low persistence phosphors (e.g.. P-3l and with a highenough frame rate. there is no flicker.

A similar technique is used to control the formation of a 4-secondsegntent of the ECG signal at the bottom of the display. Samples of thelast 4 seconds (in real time) of the ECG signal are temporarily stored,When an abnormal condition is detected (automatically, by use ofconventional prematurity detectors. for example). the samplesrepresenting four seconds of the ECG signal are transferred to a secondrecirculating shift register. Thereafter, the data stored in this secondshift register are used to form a display at the bottom of the screen.The display is formed continuously until another abnormal condition isdetected. at which time the samples representative of a new 4-second ECGsignal segment are transferred to the second recirculating shiftregister. This new 4-second ECG signal segment is then continuouslytraced out on the CRT. in this case. too. the trace which is formed onthe CRT is completely independent ofthc rate at which the ECG signal isanalyzed by the system.

Further objects, features and advantages of the invention will becomeapparent upon consideration of the following detailed description inconjunction with the drawing, in which:

l-'l(i. 1 depicts the form of the display which is acltie\ ed inaccordance with the principles of my in vention;

HUS. 2. 3 and 4. placed from left to right. are a .sche maticrepresentation of the illustrative embodiment of the in\ entionl Flti. 5depicts the counting chain for deri\ing the se\en clocl-v signalsrequired hy the system of FIGS. 2-4:

Fl(i. 6 depicts several waveforms hich will he help ful in understandingthe manner in which two different displays are formed on the CRT. and

Fl(i. 7 depicts various timing waveforms which will he helpful inunderstanding the operation of the illustrative emhodiment of theimention As shown in FIG. I. the display which is formed on the face ofthe (RT consists of two parts. The upper part is the trace of an FCG wa\ eform. In the illustrative emhodnnent of the in\ cation. 22? ECGwaveforms are traced out each second. Since the tape is played hack at(it) times the recording speed. in the usual case there are sliglttl}more than (ill ECU waveforms read from the tape per second. Depending onthe exact rate at which ECU wmeforms are read from the tape. eachwaveform is displayed several times. The frame rate is fast enough suchthat men though the persistence of the screen phosphor is so low thateach individual trace persists for less than H215 second. there is noiliclvcr hecanse all of the successive wtncforms hlend together in theohserver's eye.

Vt hcnever a premature heat is detected. it is traced out se\eral timesin rapid succession. as in the case of all other waveforms. and althoughthe traces are seen for only a hrief interval. il the superimposedtraces are different. the review er is immediately informed of theahnormal condition. At the hottom of the display of Fl(i I. there isshown a typical -l second ECG signal segment which includes a prematureheat. As will he descrihed in detail below. whenever a premature heat isdetected. the 4seeond segment containing that heat is displayed at thehottom of the ('RT. This is accomplished automatically even if theoperator misses the momentary out of-place premature heat in the upperdisplay The lower trace persists indefinitely until the next prematureheat is detected. at which time a new 4-.second segment is displayed Thelower trace is also formed at a ZIS-per-second frame rate. Since in theusual case. premature heats do not innnediately follow each other. thelower 4-second segment may he traced out many. many times before it ischanged.

The system of FIGS. 2-4 requires several timing signals. Various gateinputs are shown connected to respective clock waveforms; the timingofthe system will he evplained in detail helow. Before considering theil lustrative emhodiment of the invention. however. it should he notedhow all of the necessary clock waveforms may he derived. As shown inFIG. 5. the basic system clock is a 3.6864 MH7 oscillator. depicted bythe numeral 500. A conventional divider chain. consisting of dividers502. 504. 506. 508. 510 and 512. is used to derive six other clockwaveforms as is known in the art. Each divider element in the chaincauses its output to change state only in response to a negative step inits input. Thus any transition in the waveform of a clock signal alwaysoccurs when all higher-frequency clock signals exhihit negativetransitions.

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Fl(i. 2 depicts symholically a tape playback unit 200 hose output isapplied to the input of a play-hack ainplifier 216. The amplified ECGsignal. whose rate in the illustrative emhodiment ofthc invention is 60times the real time rate. is applied to the input of R-wave detec' tor244 and comparator 2l8. The minus input of the comparator is connectedto the output of digital-toanalog converter 2.24. The purpose of thisconnection will he descrihed below. but at this point it is sufficientto understand that the output ofthe comparator is positive \vhene\ erthe instantaneous amplitude of the signal at the plus input is greaterthan that at the minus input. and that the output of the comtiarator isnegative when the opposite condition exists. The R-wave detector 244does not per se form a part of the present invention. It simplyfunctions to trigger oneshot multivihrator 246 whenever an R wave in thecontinuou E((i signal is detected. Any conventional R-wavc detector mayhe used: one such R-wave detector is shown in Harris US. Pat. No. 3.4)(l.t-il l. issued on July (i. l97l and entitled EleetrticardiographicR-Wave Detector." Rvvave detection is required in the system because itis the detection of each R wave that initiates transfer of a waveform toa recirculating memory for suhsequent display.

The F.((i signal is sampled and it is the digital representations of thesamples which are temporarily stored. l The display itself is actuallyformed hy recirculating digital samples and converting them to analogform. followed hy the application of the analog signal to the verticaldeflection plate of the CRT.) The technique for com erting each analogsample of the ECG signal to an Whit hinary value is a standard oneinvolving the use of a successive approximation register 220 (e.g.. chipNo. AM 2502 a digititl-tdiiiiitltig converter 224 (eg. Hyhrid Circuitschip No. 37l-8 and a standard coinparator 218. The register 220. insuccessive clock cy cics. increases or decreases the binary value at itsoutputs in a direction determined by the potential level at theapproximation control" input of the element. The 8-bit value isconverted to analog form by converter 224. and the analog signal isapplied to the minus input of comparator 218. The comparator functionsto determine whether the instantaneous value of the ECG signail isgreater or less than the value represented by the 8-bit output ofregister 200. and the polarity of the comparator output determineswhether during the next clock cycle the register 220 increases ordecreases its output value in an attempt to cause the output ofconverter 224 to match the instantaneous value of the ECG signal. Itrequires nine clock pulses at the clock input of converter 220 tocontrol the Xhit output of the register to properly represent theinstantaneous value of the ECG signal which is to he sampled.

The sampling rate is I44 kHz. Although 14.400 samples are thus takeneach second. since the tape is played back at 60 times recording speed.the effective sampling rate of the ECG signal (translated to real time]is 240 samples per second. As is known in the art. this is a high enoughsampling rate so as not to lose any important information in the ECGsignal.

Successive approximation register 220 is triggered when a negative stepis applied to its com/en" input. Thereafter. nine clock pulses arerequired at its clock input until the 8 hits at the outputs Bil-B7represent the binary value of a sample. The convert input of theregister is triggered at a 14.4 RH; rate. The clock pulses occur at amuch higher rate 460.8 kHz so that each hinary sample is fully derivedlong prior to the next sampling cycle.

The vvaveforms which characterize the operations of elements 202 and204. as vvell as register 220. are shovvn at the top of HG. 7. The l4.-ikHz clock signal is applied directly to the K input of .l/K tiip-llop202. and through inverter 23" to the .l input. The 460.8 kHz clocksignal is applied to the clock input ofthe flip-flop. and the flip-flopchanges state on a negative transition in the clock signal vvhenever thestates of the .l and K inputs have changed. Since a negative transitionoccurs in the 460.8 kHz clock ignal prior to a transition in the l-L-lkHz clock signal (due to delays in successive dividers in the countingchain of Fl(i. 5 the state of flipflop 202 changes only on the negativestep in the clock signal which follovvs a change in the l-1.4 kHrsignal. This is shown by the three upper \va\eforms of FIG. 7. The third\vaveform shovvs the state of the 0 output of the flip-flop and it isapparent that it follovvs the 14.4 ltl-ly clock signal. with an oppositepolarity and after a delay of one cycle of the 460.8 kHl signal. The HAkH7. clock signal is applied to one input of gate 204. and the 0 outputof the flip-flop is applied to the other inputv Only when hoth inputsare high is the output of the gate lovv. Thus follovving each positivestep in the l4.-l RH? ClUCis signal. a short negative pulse appears atthe output of gate 204. This negative pulse. applied to the convertinput of register 220. triggers a successive approximation cycle. The460.8 kHy clock pulses are applied through inverter 242 to the clockinput of the register. and after nine clock cycles the 8-bit output ofthe register represents the binary value of the instantaneous magnitudeof the ECG signal at the output of amplifier 216. hi the fifth line ofFl(i. 7. the vertical arrows represent the completion of each conversioncycle. that is. the availahility ol'a digital sample at the outputs ofregister 22".

Digital multiplexer 226 is provided with two groups of eight inputseach. Bits Bit-B7 at the outputs of register 220 are extended to inputset A. while input set B is groundedv The multiplexer operates to extendrespcctive hit values in a selected one of the tvvo input groups to itseight outputs M(IM7 in accordance with the polarity of the signal at theSELECT A input. When this input is low. the signals (ground) at the Binputs are extended to the multiplexer outputs'. when the SE- LECT Ainput is high. hits B(IB7 are extended to the multiplexer outputs.

The eight multiplexer outputs are connected to the data inputs ofrespective 256-bit shift registers 228-0 through 228-7. The output ofgate 258 is connected to the shift input of each shift register. As willbe described shortly. as long as the SELECT A input of multiplexer 226is high. successive shift pulses from gate 258 control the storage ofsuccessive 8-bit samples in the eight shift registers. Shift pulses aregenerated at a 14.4 kHz rate. there thus being one shift pulse for eachnew sample (at the multiplexer outputs) which is taken of the ECGsignal.

The shift registers thus represent 25h 8-hit samples of the ECG signal.As descrihed hriefly above. all of this data is rapidly transferred to arecirculating memory (another set of8 shift registers) for subsequentdisplay. Since incoming ECG waveform data is shifted along the shiftregisters 228-!) through 228-7 it is important to synchronize the rapidtransfer of the data to the recirculating memory at a time which permitseach ECG Ill waveform to he properly placed on the display tFl(i. l l.

The R-\va\e detector 244 detects the presence of each vvavelorm. Buteach vvaveform is detected \v hen the samples representative ofthe Rwave are still stored at the input end of the shift registers. Ratherthan shifting the data out of the shift registers as soon as an R waveis detected. it is preferahle to allow the data to he shifted dovvn theregisters until the R vvave is centered: this. in turn. permits theo\erall waveform to he centered on the display. Accordingly. when theR-\vave detector 244 verifies the pre ence of an R a\e. one-shott'ltLllliYihlltlUl 246 is triggered. It is only at the trailing edge ofthe positive output pulse of this multivihrator that one-shotmultivihrator 250 is triggered. lt is hen the output of thismultivihrator goes high that the system rapidly transfer the data inshift registers 228-0 through 228-7 to the recirculating memory used toform the display. Potentionieter 248 is used to control the delayhetvveen the detection of an R wine and the start of the rapid transferof data out of the shift registers. It is on the leading edge of thepulse at the output of one-shot niultiviln'ator 250 that the transferbegins.

The entire transfer takes place in less than one period of the l4.-1 kH/clock. as \vill he explained in connection with the timing vvavetorms ofFl(i. 7. The duration of the pulse generated hy one-shot multivihrator250 must he at least as great as one period of the l-1.-t kHz ClULhsignal. but it should not he excessively long or else data will hetransferred out of the shift registers at the fast rate vvhen it shouldhe stored in them at the slovv rate l4.4 l-v'HY t. For this reason. inthe illustrative embodiment of the invention. the duration of the pulseat the output of multivihrator 250 is l5 periods of the l4.4 kH7 clocksignal.

As shown in FIG. 7. the Q output of Hip-Hop 252 is normally low. The 0output of the flip-flop is fed hack to the K input. Since the .l input(output of multivihrator 250) is normally lovv. the l4.4 kH7eloclspulses applied to the clock input of the flip-flop have no effectsince if hoth inputs to a UK flip-flop are lovv. the flipflop state doesnot change with the application of clock pulses. But as soon as theoutput of multivihrator 250 goes high. as shown in FIG. 7, the flip-flopchanges state with its 0 output going high on the next negativetransition at the clock input. This occurs at a falling edge of the 14.4kHz. clock \vaveform. As soon as the flip-flop changes state. the Kinput goes high along with the J input. The next negative step in theclock input causes the flip-flop to change state once again (a J/Kflip-flop changes state upon the application of a clock input wheneverboth of the .l and K inputs are high). Thus. as shown in FIG. 7 it is atthe trailing edge of the next l4.4 kHz clock pulse that the 0 output offlip-flop 252 goes low once again. It remains low until another R waveis detected. that is. until after approximately another 256 samples havebeen taken. lt is during a single period ofthe 14.4 kHz clock signal.while the 0 output of flip-flop 252 is high. that 256 shift pulses areapplied to the shift input of registers 228-0 through 228-7 at a veryhigh rate in order to rapidly transfer all of the data to the recirc ulating memory.

Ordinarily. the Q output of flip-flop 252 is high. and this output iscoupled to one input of gate 256. The l4.4 l\'H'/. clock signal isapplied through inverter 222 to the other input of gate 256.Consequently. the output of gate 256 follows the l4.4 kHz clock signal.as indicated in l-'l(i. 7. The output of gate 256 is connected to oneinput of gate 258. Ordinarily. the output of gate 254. one of whoseinputs is connected to the 0 output of flip-flop 252. is high. Thisoutput is connected to the second input of gate 258 and has no effect onits output. Consequently. as long as llip-tiop 252 has itsU output high.it is only gate 256 that controls the application ofshift pulses to theshift registers. The 14.4 kH/ clock signal is extended through gate 258.but is in- \ertcd. The output of gate 258 is thus the complement of the1-1.4 kHY clock signal. Each of shift registers 228-0 through 228-7executes a shift operation when a positive step is applied to its shiftinput. that is. when the output of gate 256 goes low. This is depictedin Fl(i. 7 h the \ertical arrows in the line labeled SHll-"l [SLOW It isthe falling edge of each waveform in the l4.-l lsH/ clocls cycle thatcontrols the shifting of data in the eight hift registers.

During the time that the 0 output of flip-flop 252 is low. the SIZLFCT Ainput of multiplexer 226 is high. thus allowing hits 80-87 from register220 to be extended through the multiplexer to the shift register inputs.it is in this way that a new sample is stored in the shift registersduring each ofthe l4.-1 kHv clock cycles. It should he noted from Fl(i.7 that each sample is a\ailable at a time indicated by the arrows in theline labeled "CONVERSION COMPLETE". each sample being stored in theshift registers shortly thereafter with the generation of a SHIFT (SLOW)signal.

But whenever an R wave is detected. and following the delay introducedby one-shot multivibrator 246. flip-flop 252 changes state. At this timethe 6 output goes low so that the inputs to the shift registers 228-0through 228-7 represent eight 0's. With the 0 output high. gate 254transmits 3686.4 kHz clock pulses through it: these pulses pass throughgate 258 to the shift register shift inputs. This operation is shown bythe line labeled (iA'l'E 254" in Fl(i. 7. The output of the gate isordinarily high but as soon as flip-flop 252 changes state. the 3686.4kHz pulses are extended through it. These pulses are extended throughgate 258 to the shift inputs of the eight shift registers. Just as ashift operation is controlled by the output of gate 256 going low, so ashift operation is controlled by the output of gate 254 going low. Thebottom line of FIG. 7 labeled SHIFT tFASTl" depicts the fast shiftoperations. Since fast shift pulses are generated during one completecycle of the l4.4 kHz signal. and since the fast shift pulses (whichoccur at at 3686.4 kHz rate) occur at a rate which is 256 times as fastas the slow shift pulses. it is apparent that all 256 hits in each shiftregister are shifted out on conductors 270-0 through 270-7 during onecycle of the 14.4 ls'Hvclock signal. as indicated in FIG. 7. At the endof this cycle. there are 256 0's stored in each shift register inasmuchas the data input to each shift register represents a 0 since the SELECTA input of multiplexer 226 is low throughout the duration of the fastshift operation. Immediately following the 256 fast shift pulses.flip-flop 225 changes state once again and successive samples are storedin the shift registers at the slow rate.

It should be noted that a slow shift pulse is generated shortly aftereach sample conversion except for that sample conversion whichimmediately precedes the 0 output of flip-flop 252 going high. Thus onesample of the ECG waveform is actually lost. that is. not stored inshift registers 228-0 through 228-7 during that cycle fit of the 1-1.4l'sHl clock signal in which the pre\iously stored samples are rapidlytransferred out of the shift registers. This is of no importance.particularly since the lost sample" occurs between successive F('(i w a\eforms (along the base line). It should also be noted that the number ofhits actually shifted out of the shift register depends upon the numberof samples which are taken between successive R waves. This number canvary although it is more or less constant as long as normal iii-stepwaveforms are present.

Another set of lib-hit shift registers 304-0 through 304-7 is providedon FIG. 3. The samples which are shifted out of registers 228-0 through228-7 at at 3686.4 kHzv rate are stored in registers 304-0 through304-7. Thereafter. the samples are recirculated in these registers atthe same time that they are used to form the upper trace on the display.The recirculating rate is much slower than the high-rate transfer.

It is when the 0 output of flip-flop 252 is high that the fast transferis to take place. Conductor 262. which is connected to the Q output ofthe flip-flop. is extended to the SELECT A input of digital multiplexer302. The outputs of shift registers 228-0 through 228-7 are connected tothe eight inputs in input set A of multiplexer 302. Consequently. duringthe fast transfer. the samples which are being transferred betweenregisters are extended through multiplexer 302 to the inputs ofshiftregisters 304-0 through 304-7. During this transfer, clock pulses appearon conductor 260 at at 3686.4 kHz rate. These pulses are extendedthrough gate 308 to the shift inputs of register 304-0 through 304-7.Consequently. the 256 shift pulses which are generated during a singlecycle of the l4.4 kHz waveform control the storage of all datarepresentive of a single ECG waveform in registers 304-0 through 304-7.At the end of the fast transfer. conductor 260 remains high and has noeffect on the shifting in registers 304-0 through 304-7. And conductor262 which now goes low causes the data at the eight inputs in set B ofmultiplexer 302 to be extended through the multiplexer to shiftregisters 304-0 through 304-7. The eight output conductors D0-D7 fromthese registers are extended hack to respective inputs of set B ofmultiplexer 302. Consequently. shift pulses applied to the registerssimply control the recirculation of the data stored in them.

The shift pulses which control the r ecirculation are derived from gate306. As long as the 0 output of fliptlop 252 is high (during the slowstorage of samples in shift registers 228-0 through 228-7. and duringthe recirculation of data in shift registers 304-0 through 304-7).conductor 266 is high in potential. This conductor is connected to oneinput of gate 306 and the l l5.2 kHz clock signal is connected to theother input of the gate. Consequently. I 15.2 kHz clock pulses areextended through the gate and through gate 308 to the shift inputs ofregisters 304-0 through 304-7 to control the recirculation of the data.(The data which are shifted out of the registers on conductors D0 D7 arealso extended to the circuitry on FIG. 4 which is used to form thedisplay. as will be described below.)

The recirculation rate is controlled by the l l5.2 kHz clock signal. andeach waveform is represented by 256 bits in registers 304-0 through304-7. It is thus apparent that in each second there are t 15.200/256 or450 complete rccirculations ofdata. As will be described below, an ECGwaveform trace is developed only during alternate recirculations. andconsequently an ECG waveform is traced out on the display 115 times eachsecond. Since ECU waveforms are actually processed from the tape at therate of approximately 60 per second. it is apparent that each 13((1waveform is traced out on the CRT several times.

The circuitry descrihed thus far functions to temporarily store eachF.((i waveform and to then quickly transfer it to a recirculating memoryfrom which a trace may he developed. In a similar manner. provision ismade for temporarily storing approximately 4 seconds of the ECG signal[represented hy L024 R-hit samples). and following the detection of apremature heat to quickly transfer this data to another lllZ-l-samplerecirculating memory to control the continuous display of a 4-second(real time) ECU signal segment at the hottom of the display. tlnstead of4 seconds of storage. it is also contemplated that a signal including asfew as three successive ECU waveforms he stored.)

The X-hit samples at the output of register 220 are extended to theeight inputs in set A of multiplexer 310. The eight inputs in set B aregrounded and the eight outputs of the multiplexer are extended to theinputs of the eight l024-hit shift registers 312-0 through 312-7.Multiplexer 310 and registers 312-0 through 3l2-7 are analogous tomultiplexer 226 and shift registers 228-0 through 228-7. Data isnormally stored in registers 312-0 through 312-7 at a 14.4 kHz rate.hrit when a premature heat is detected and the data in the registers areto he transferred out to a recirculatirtg memory at a fast rate. the3686.4 kHz clock is employed.

The output of R-vvavc detector 244 is extended over conductor 264 to theinput of prematurity detector 314. This detector. having a time constantof 6 seconds (real time). functions to average the time interval hetweensuccessive pairs of R waves. and to energize its output if any R wave ispremature hy ltl percent Any of many prematurity detectors can heemployed. and one such prematurity detector is disclosed in Harris US.Pat. No. 3.6l6,79l entitled ELFCTROCARDIO- GRAPHIC MORPHOLOGYRECOGNITION SYS- TEM" dated Nov. 2. 1971. Rather than to detect onlypremature heats. it is also possihle to detect an ahnorrnal morphologyas is disclosed in the last-mentioned patent for the purpose oftriggering a new 4-second segment display whenever any ahnormal heat isdetected (even if it is not premature).

When a premature heat is detected. one-shot multivihrator 316 istriggered The period of this multivihrator is controlled bypotentiometer 338. the potentiometer being set so that the prematureheat will he displayed approximately at the center of the display. (Thatis. the fast transfer out of registers 312-0 through 312-8 is delayed byapproximately 2 seconds in real time. or 2/60 seconds in processing timet. At the trailing edge of the pulse at the output of multivihrator 316.multivihrator 318 is triggered for controlling the fast transfer.

The 0 output of flip-flop 320 is normally low and the 0 output isnormally high. Consequently. 14.4 kHz clock pulses are extended throughinverter 342 and gate 336 to one input of gate 322 to control thestorage of samples in registers 312-0 through 312-7 at the same rate asthe rate at which they are taken. namely. 14.4 kHz. When one-shotmultivihrator 318 is triggered. however, the J input of flip-flop 320goes high and the flip flop changes state just as flip-flop 252 changesstate when one-shot multivihrator 250 is triggered.

Ill

As soon as flip-Hop 320 changes state ton a negative step in the 3.6 kHyclock signall. it is gate 334 vvhich transmits clock pulses from the3686.4 kH7 clock through gate 322 to the shift registers. rather thangate 336 transmitting l-L-l kH/ clock pulses through the same gate 322.Also. the SELECT A input of multiplexer 310 goes low so that followingthe fast transfer out of the shift registers. each shift registercontains Nil-10's. Flip-flop 320 controls the fast transfer during fourcomplete cycles of the 1-1.4 kH/ clock signal since the clock input offlip-flop 320 is connected to the 3.6 kHl clock signal. While the clocksignal for multivihrator 252 is 14.4 kHV. the clock signal formultivihrator 320 is 3.6 kHz That is hecause there are 4 times as manyhits stored in each of registers 312-0 through 312-7 than there arestored in each of registers 228-0 through 228-7. The storage of samplesin the 1024-hit shift registers and the fast transfer of the samples outof the registers is analogous to the storage of samples in the 256-hitregisters and the fast transfer of the hits out of the registers. 1During the fast transfer. four samples from register 220 are "lost thatis. not stored in registers 312-0 through 312-7 hat that is of littleconccrn.l

lit a similar manner. multipleser 400 and shift registers 402-0 through402-7 control the fast storage of 1024 samples in the shift registers.follovv ed hy their recirculation. During the fast transfer. conductor326 is high and the SELECT A input of multiplexer 400 is high. At thistime. the samples on conductors 330-0 through 330-7 which are shiftedout at a fast rate from registers 312-0 through 312-7 are stored inshift registers 402-0 through 402-7. The shift pulses for the shiftregisters pass through gate 406 and are derived from conductor 324. Theclock pulses used to control the storage of data in registers 402-0through 402-7 are those used to control the transfer of data out ofregisters 312-0 through 312-7.

Following the fast transfer. conductor 324 remains high in potential andhas no effect on the shifting of hits in registers 402-0 through 402-7.Similarly. conductor 326 switches to a low potential so that the eightinputs in set B of multiplexer 400 are selected for extension to theoutputs. Consequently. data shifted out of registers 402-0 through 402-7are recirculated in a manner comparable to the recirculation of datacontrolled by multiplexer 302 and shift registers 304-0 through 304-7.Since conductor 328 is high in potential at all times other than duringa fast transfer of samples from one set of shift registers to the other.gate 404 functions to extend 460.8 kHz clock pulses through gate 406 tothe shift inputs of registers 402-0 through 402-7. It should he notedthat the rate at which data is recirculated in registers 402-0 through402-7 is 4 times as great as the rate at which data is recirculated inregisters 304-0 through 304-7. That is hecause there are 4 times as muchdata stored in the former set of registers than in the latter. By usinga clock which is 4 times as fast, there is a complete recirculation of 4seconds of data in registers 402-0 through 402-7 in the same time thatthere is a complete recirculation of one second of data in registers304-0 through 304-7. in both cases. a complete CRT line of data isrecirculated at a rate of 450 recirculations per second. Since the lowertrace on the CRT is formed during only every other recirculation. thelower trace is formed at a rate of 225 per second. (As will he describedbelow. the upper end and lower traces are formed during alternatehorizontal sweeps. I

The samples shifted out of registers 402- through 402-7 are applied tothe eight inputs in set A of multiplexer 418. The data which are shiftedout of registers 304-0 through 304-7 are applied to the eight inputs inset 8 of multiplexer 418. The SELECT A input of multiplexer 4|?! changespolarity at a rate of 450 per second so that during alternate horizontalsweeps either 250 samples are extended through the multiplexer tocontrol the formation of the upper CRT trace. or i024 samples areextended through the multiplexer to control the formation of the lowerCRT trace.

Since data for each of the two displays is recirculated at a rate of 450complete recirculations per second. and each display is formed inalternate cycles. 11225 Hz clock signal is required for alternatelycontrolling the use ofthe samples shifted out ot either ofthe tworecirculating memories. Multiplexer 4l8 is utilized for alternatelycontrolling the formation of a display from either 256 samples which arerecirculated in one memory or from i024 samples which are recirculatedin the other. The 225 Hz clock signal is extended through iii- \erter4l2 to the SELECT A input of the multiplexer.

During each half cycle that the SELECT A input of the multiplexer ishigh. I024 samples from shift registers 402-0 through 402-7 are extendedthrough the multiplexer to the inputs of 8-bit latch 422. It will berecalled that data is recirculated in shift registers 402-0 through402-7 under control of a 460.8 kHz clock. Since there are I024 samplesstored in the shift registers. complete recirculation of the data occursat a 450 Hz rate (460.80tl+ LUZ-1:450). Similar remarks apply to therecirculation of data in shift registers 304-0 through 304-7. data beingshifted at one-quarter the rate so that complete recirculations of dataalso occur at a 450 Hz rate. lt is thus apparent why a 225 Hz clocksignal is used to switch between the two sets of inputs in multiplexer4l8; there is a complete recirculation ofeach set of data during eachhalf cycle of the 225 Hz clock.

Each sample which is extended through the multiplexer is stored in 8-bitlatch 422. The latch must he strobed at the same rate at which newsamples are extended through the multiplexer. When the 225 Hz clocksignal is high. it is the samples from shift registers 304-0 through304-7 which are extended through the multiplexer to the latch. at a H52kHz rate. For this reason. the latch must be strobed at this rate. Gate410 is enabled when the 225 Hz clock signal is high. and H52 kHz clockpulses are transmitted through inverter 408. gate 410 and gate 420 tothe strobe input of the latch. in a similar manner. during alternatecycles. it is gate 4l6 which is enabled and it is now the 4608 kHz clocksignal which is extended through inverter 414. gate 416 and gate 420 tothe strobe input of the latch. ln both cases. the latch is strobed onthe falling edges ofthe I 15.2 kHz and 460.8 kHz clock signals. toensure that a sample extended through multiplexer 418 has had time tosettle.

The output of the latch is extended to the input of digital-to-analogconverter 424. and the output of this element is an analogrepresentation of the signal to be traced out on the display.

The output of the digital-to-analog converter 424 is not extendeddirectly to the vertical input ofCRT 430. instead. it is extendedthrough a resistor 432 to a summing junction. The second input to thesumming junction is a potential source 438 which is extended through apotentiometer 436. The potentiometer simply controls the verticalposition of the entire display. a technique well known in the art. Thethird input to the summing junction is the 225 Hz clock signal which isextended through potentiometer 434. During alternate half cycles of theclock signal. the vertical deflection signal is increased so as to allowthe two different displays to be formed at different levels on the faceof the CRT.

FIG. 6 depicts the relevant timing waveforms for the CRT. At the top ofthe drawing there are shown the 450 Hz and the 225 Hz clock signals. itwill be recalled that when the 225 Hz clock signal is high the SELECT Ainput of multiplexer 4T8 is low so that it is the samples required forthe upper trace which are extended to latch 422. It is at this time thatthe vertical deflection signal is increased by the 225 Hz clock signalextended through potentiometer 434. with the output of thedigital-to-analog converter 424 being positioned on the base linedefined by the 225 Hz clock signal. This is shown in the bottom waveformof FIG. 6. During alternate half cycles of the 225 Hz clock signal. whenthe SELECT A input of multiplexer 418 is high and the lower trace is tobe formed on the display. the 225 Hz input to the summing junction islow and the output of the digital-to-analog converter 424 is positionedon the lower base line. [The additional bias through vertical positioncontrol potentiometer 436 simply moves the entire display up or down onthe screen; the setting of potentiometer 434 controls the space betweenthe two separate traces.)

As described above. horizontal sweeps must occur at a 450 Hz rate. twohorizontal sweeps being required during each cycle ofthe 225 Hz clock sothat one trace may be made for each display. The 450 Hz clock signal isextended to the input of one-shot multivibrator 426. The short (retrace)pulse output from this multivibrator, shown in FIG. 6. is used for twopurposes. First. it triggers the horizontal sweep circuit 428. theoutput of which is extended to the horizontal deflection circuit of theCRT. Any standard horizontal sweep circuit may be utilized for thispurpose. and the horizontal sweep waveform is shown in FIG. 6. Second.since the triggering of the horizontal sweep circuit first results inthe retrace of the signal. followed by a sweep. it is desirable to blankthe CRT during the retrace. For this reason. the output of one-shotmultivibrator 426 (the retrace multivibrator) is extended to theblanking input of the CRT to shut off the electron beam during retrace.

it is thus apparent that the CRT display is completely independent ofthe tape speed. Once data is stored in the recirculating memories. acontinuous display will be formed even ifthe tape is stopped altogetherand no new input data is examined. The recirculation rate. since it isindependent of tape speed. can be selected in any particular system, inconjunction with the particular phosphor which is used in the CRT. toprovide the best possible display for the purposes of the system. Thequality of the display does not change even if the tape speed is varied.Furthermore. the automatic formation of the lower display. which depictsevery 4- second segment of the ECG signal which contains a prematurebeat, ensures that no premature beat goes undetected.

Although the invention has been described with reference to a particularembodiment. it is to be understood that this embodiment is merelyillustrative of the application of the principles of the invention.Numerous modifications may he made therein and other arrangements may bedevised without departing from the spirit and scope of the invention.

What I claim is:

l. A system for reviewing an ECG signal stored on a recording mediumcomprising means for reading the ECG signal from said recording mediumat a rate substantially faster than the rate at which said ECG signalwas recorded. first means for temporarily storing successive ECGwaveforms read from said recording medium. first recirculating memorymeans. means responsive to the storage of an ECG waveform in said firsttemporary storage means for transferring the stored ECG waveform to saidfirst recirculating memory means. second means for temporarily storing asegment of the ECG signal read from said recording medium which is atleast long enough to include 2 normal heartbeat cycles secondrecirculating memory means, means for detecting an abnormal heartbeat inthe ECG signal read from said recording medium and responsive theretofor transferring the temporarily stored ECG sig nal segment to saidsecond recirculating memory means. means for recirculating an ECGwaveform stored in said first recirculating memory means and an ECGsignal segment stored in said second recirculating memory means at ratesfaster than the rates at which an ECG waveform and an ECG signal segmentare stored in said first and second temporary storage means. and displaymeans synchronized to the recircu' lation rates of said first and secondrecirculating memory means for displaying all ECG waveforms stored insaid first recirculating memory means superimposed on each other.whereby ECG waveform traces are formed on said display means at a ratefaster than that at which they are read from said recording medium. andfor displaying separately the ECG signal segment stored in said secondrecirculating memory means.

2. A system in accordance with claim I wherein ECG waveforms and ECGsignal segments are stored in said first and second temporary storagemeans and in said first and second recirculating memory means in theform of samples.

3. A system in accordance with claim 2 wherein the ECG waveform storedin said first temporary storage means is transferred to said firstrecirculating memory means at a rate so fast that during the transfertime there is an insignificant change in the ECG signal read from saidrecording medium.

4. A system in accordance with claim 2 wherein the ECG signal segmentstored in said second temporary storage means is transferred to saidsecond recirculating memory means at a rate so fast that during thetransfer time there is an insignificant change in the ECG signal readfrom said recording medium 5. A system in accordance with claim 2wherein said display means includes a CRT. means for converting thesamples recirculated in said first and second recirculating memorymeans. during respective recirculation cycles thereof. to derive avertical deflection signal for said CRT. means for synchroniying thehorizontal sweeps of said CRT to the recirculation rates of said firstand second recirculating memory means during respective recirculationcycles thereof. and means for changing the bias of said verticaldeflection signal for respective ones of said recirculation cycles sothat two separate displays are formed on said CRT.

6. A system in accordance with claim 2 wherein said abnormal heartbeatdetecting means is a prematurity detector.

7. A system in accordance with claim I wherein the ECG waveform storedin said first temporary storage means is transferred to said firstrecirculating memory means at a rate so fast that during the transfertime there is an insignificant change in the ECG signal read from saidrecording medium.

8. A system in accordance with claim I wherein the ECG signal segmentstored in said second temporary storage means is transferred to saidsecond recirculating memory means at a rate so fast that during thetransfer time there is an insignificant change in the ECG signal readfrom said recording medium.

9. A system in accordance with claim I wherein said abnormal heartbeatdetecting means is a prematurity detector.

1. A system for reviewing an ECG signal stored on a recording mediumcomprising means for reading the ECG signal from said recording mediumat a rate substantially faster than the rate at which said ECG signalwas recorded, first means for temporarily storing successive ECGwaveforms read from said recording medium, first recirculating memorymeans, means responsive to the storage of an ECG waveform in said firsttemporary storage means for transferring the stored ECG waveform to saidfirst recirculating memory means, second means for temporarily storing asegment of the ECG signal read from said recording medium which is atleast long enough to include 2 normal heartbeat cycles, secondrecirculating memory means, means for detecting an abnormal heartbeat inthe ECG signal read from said recording medium and responsive theretofor transferring the temporarily stored ECG signal segment to saidsecond recirculating memory means, means for recirculating an ECGwaveform stored in said first recirculating memory means and an ECGsignal segment stored in said second recirculating memory means at ratesfaster than the rates at which an ECG waveform and an ECG signal segmentare stored in said first and second temporary storage means, and displaymeans synchronized to the recirculation rates of said first and secondrecirculating memory means for displaying all ECG waveforms stored insaid first recirculating memory means superimposed on each other,whereby ECG waveform traces are formed on said display means at a ratefaster than that at which they are read from said recording medium, andfor displaying separately the ECG signal segment stored in said secondrecirculating memory means.
 2. A system in accordance with claim 1wherein ECG waveforms and ECG signal segments are stored in said firstand second temporary storage means and in said first and secondrecirculating memory means in the form of samples.
 3. A system inaccordance with claim 2 wherein the ECG waveform stored in said firsttemporary storage means is transferred to said first recirculatingmemory means at a rate so fast that during the transfer time there is aninsignificant change in the ECG signal read from said recording medium.4. A system in accordance with claim 2 wherein the ECG signal segmentstored in said second temporary storage means is transferred to saidsecond recirculating memory means at a rate so fast that during thetransfer time there is an insignificant change in the ECG signal readfrom said recording medium.
 5. A system in accordance with claim 2wherein said display means includes a CRT, means for converting thesamples recirculated in said first and second recirculating memorymeans, during respective recirculation cycles thereof, to derive avertical deflection signal for said CRT, means for synchronizing thehorizontal sweeps of said CRT to the recirculation rates of said firstand second recirculating memory means during respective recirculationcycles thereof, and means for changing the bias of said verticaldeflection signal for respective ones of said recirculation cycles sothat two separate displays are formed on said CRT.
 6. A system inaccordance with claim 2 wherein said abnormal heartbeat detecting meansis a prematurity detector.
 7. A system in accordance with claim 1wherein the ECG waveform stored in said first temporary storage means istransferred to said first recirculating memory means at a rate so fastthat during the transfer time there is an insignificant change in theECG signal read from said recording medium.
 8. A system in accordancewith claim 1 wherein the ECG signal segment stored in said secondtemporary storage means is transferred to said second recirculatingmemory means at a rate so fast that during the transfer time there is aninsignificant change in the ECG signal read from said recording medium.9. A system in accordance with claim 1 wherein said abnormal heartbeatdetecting means is a prematurity detector.